The invention is particularly useful in the field of microelectronics. Indeed, in this field, the manufacturing techniques such as photolitohography and etching are in rapid progress and patterns having a width smaller than 1 micrometer are obtained. Thus, when they study an integrated circuit, the designers define the acceptable margins for the patterns that are used for localizing the various functions. The size differences are generally due to not controlled variations of physical values that occur during the manufacturing. Tests are provided for detecting the manufacturing steps which are problematic and for quickly adjusting the machines causing such defects. However, such tests slow down the manufacturing and increase the cost thereof and it is important to avoid erroneous decisions which cause the disposal of correct circuits or the acceptation of deficient circuits. Therefore, the result of controls has to be correct and reliable.
The largest part of the systems for dimensional measurement operate on an enlarged image of the pattern to be measured. This image is provided through an optical or electronic microscope to which is associated an equipment specially provided for ensuring the measurement function.
The microscope constitutes the first stage of the measuring chain. It comprises an irradiating device, an object-carrying plate, and an imaging device.
The irradiation is obtained through an electron source in case of a scanning electron microscope (SEM) or through a light source (lamp or laser) in case of optical microscopy. Once irradiated, the various regions of the object either emit secondary electrons (SEM) or reflect a portion of the incident light. The interaction between the irradiation and the object depends locally on the nature and the arrangement of the considered materials. The resulting phenomenon is itself modified by the imaging device which collects the information for transmitting same to the second measuring stage: the acquisition device.
The information acquisition is carried out through a transformation of the optical (or electronic) space signal into an electrical signal that can be digitalized. This transformation of a space information into a time information is carried out by scanning. In a SEM, the object and the detector are fixed: it is the irradiation beam that moves with respect to the object. In conventional optical microscopy, the irradiation, the object and the image are fixed: it is the scanning device of the sensor that generates the wished signal.
The size that one wishes to determine is defined as the distance between the two mechanical edges of a line. Initially, the shape of the pattern is defined by its drawing on a mask. Its structure is basically two-dimensional and into a given direction.
On the contrary, the pattern obtained on an integrated circuit wafer necessitates a three-dimensional description. This pattern is characterized by the thickness of the materials and the shape of the edges. In particular, if the edges are not vertical, and this is the common case, it is necessary to precise the level to which corresponds the measured dimension.
Due to its high resolution factor, the SEM permits to measure the topography and can provide a measurement reference. Unfortunately, the operation is destructive. Indeed, it is necessary to cleave the sample perpendicularly to the direction of the line and to observe the cross section of the pattern. Additionally, in order to avoid phenomenons due to electrical charge accumulation in the material, the object has to be metallized. Moreover, for permitting the calibration of the microscope magnification, the pattern has to belong to a recurring structure, the pitch of which is easily measured. Such a measurement method is unpractical to implement. However, it is presently the only known method for providing precise measurements of objects having a very small size. The resolution limit of the SEM apparatuses is in the range of 0.5 to 50 nm while this limit is only between 300 to 1,000 nm for the optical systems.
In order to more efficiently utilize the resolution of the optical microscope systems, the French patent application No. 83/12328 of July 26, 1983 discloses a specific method wherein, instead of directly observing the image of a line, this image is transformed into the frequency field by discrete Fourier transform. However, this method finds reliable application only when the thickness of the object to be measured is smaller than the field depth of the optical microscope. In fact, it is not possible to implement measurements on layers having a thickness higher than about 0.8 micrometer.
Accordingly, for very small size structures, having a width lower than 1 micron, but a thickness in the range of 1 micron or more, for example a line having a width of 0.8 .mu.m and a thickness of 1.2 .mu.m, no reliable and non-destructive method of measuring the width of a line is presently available. Additionally, when one considers such a structure which has not a priori vertical edges, the size measurement has no real meaning but it is additionally necessary to carry out a shape determination.